Metal-promoted coupling reactions, alkynes

Alkynes enter into a remarkable variety of metal-promoted coupling reactions with olefins, alkynes, and other unsaturated species leading to a diversity of cyclization, oligomerization, and polymerization products of synthetic value. In many instances alkyne complexes are presumed intermediates in these reactions but often this has not been proven. The reader is referred to other reviews [95-97] for more complete coverage of this topic. We briefly summarize here the most useful of these processes, highlighting those systems in which metal-alkyne complexes have been demonstrated as intermediates. 

The same transition metal systems which activate alkenes, alkadienes and alkynes to undergo nucleophilic attack by heteroatom nucleophiles also promote the reaction of carbon nucleophiles with these unsaturated compounds, and most of the chemistry in Scheme 1 in Section 3.1.2 of this volume is also applicable in these systems. However two additional problems which seriously limit the synthetic utility of these reactions are encountered with carbon nucleophiles. Most carbanions arc strong reducing agents, while many electrophilic metals such as palladium(II) are readily reduced. Thus, oxidative coupling of the carbanion, with concomitant reduction of the metal, is often encountered when carbon nucleophiles arc studied. In addition, catalytic cycles invariably require reoxidation of the metal used to activate the alkene [usually palladium(II)]. Since carbanions are more readily oxidized than are the metals used, catalysis of alkene, diene and alkyne alkylation has rarely been achieved. Thus, virtually all of the reactions discussed below require stoichiometric quantities of the transition metal, and are practical only when the ease of the transformation or the value of the product overcomes the inherent cost of using large amounts of often expensive transition metals. 

Homo- and cross-couplings of alkyne promoted by metallic catalysts are versatile reactions for carbon-carbon bond formation and have been utilized to synthesize functional hyperbranched polymers. 

Abstract In the presence of low-valent transition metal centers, CO2 can react with unsaturated organic substrates, such as alkenes, alkynes, and conjugated and cumulated dienes, to give carboxylated products. This chapter focuses on the main mechanistic features of these carboxylation processes. A key step of these transformations is the metal-promoted oxidative coupling reaction between CO2 and the unsaturated substrate. The mechanistic details of this step are highlighted and the relevance and role of CO2 coordination to metal center in these reactions is argued. 

In recent years a multitude of metal-promoted C-C coupling reactions of arene-1-alkynes to haloarenes and haloheteroarenes... 

Several cobalt compounds are widely used as oxidation catalysts. Cobalt-based catalysts are also important in some industrial process such as the Fischer-Tropsch process [9] and the hydroformylation of alkenes [10]. Also, the cross-coupling reactions promoted by this metal have been recently highlighted [11]. Conversely, cobalt is not a suitable metal catalyst for the O-H addition to alkynes, alkenes, and nitriles. 

The key success of these metal-catalyzed processes lies in the replacement of an unachievable carbozincation by an alternative carbometallation involving the transition metal catalyst, or another pathway such as an alkene-alkene (or alkyne) oxidative coupling promoted by a group IV transition metal complex, followed by transmetallation. An organozinc is ultimately produced and the latter can be functionalized by reaction with electrophiles. 

---